Nonvolatile memory circuits such as electrically erasable programmable read only memories (EEPROM) and Flash EEPROMs have been widely used for several decades in various circuit applications including computer memory, automotive applications, and video games. Each of these nonvolatile memory circuits has at least one nonvolatile memory element such as a floating gate, silicon nitride layer, programmable resistance, or other nonvolatile memory element that maintains a data state when an operating voltage is removed. Many new applications, however, require the access time and packing density of previous generation nonvolatile memories in addition to low power consumption for battery powered circuits. One nonvolatile memory technology that is particularly attractive for these low power applications is the ferroelectric memory cell, which uses a ferroelectric capacitor for a nonvolatile memory element. A major advantage of these ferroelectric memory cells is that they require approximately three orders of magnitude less energy for write operations than previous generation floating gate memories. Furthermore, they do not require high voltage power supplies for programming and erasing charge stored on a floating gate. Thus, circuit complexity is reduced and reliability increased.
The term ferroelectric is something of a misnomer, since present ferroelectric capacitors contain no ferrous material. Typical ferroelectric capacitors include a dielectric of ferroelectric material formed between two closely-spaced conducting plates. One well-established family of ferroelectric materials known as perovskites has a general formula ABO3. This family includes Lead Zirconate Titanate (PZT) having a formula Pb(ZrxTi1-x)O3. This material is a dielectric with a desirable characteristic that a suitable electric field will displace a central atom of the lattice. This displaced central atom, either Titanium or Zirconium, remains displaced after the electric field is removed, thereby storing a net charge. Another family of ferroelectric materials is Strontium Bismuth Titanate (SBT) having a formula SbBi2Ta2O9. SBT has several advantages over PZT. Memories fabricated from either ferroelectric material have a destructive read operation. In other words, the act of reading a memory cell destroys the stored data so that it must be rewritten before the read operation is terminated.
A typical one-transistor, one-capacitor (1T1C) ferroelectric memory cell of the prior art is illustrated at FIG. 1. The ferroelectric memory cell is similar to a 1T1C dynamic random access memory (DRAM) cell except for ferroelectric capacitor 100. The ferroelectric capacitor 100 is connected between plate line 110 and storage node 112. Access transistor 102 has a current path connected between bit line 108 and storage node 112. A control gate of access transistor 102 is connected to word line 106 to control reading and writing of data to the ferroelectric memory cell. This data is stored as a polarized charge corresponding to cell voltage VCAP. Capacitance of bit line BL is represented by capacitor CBL 104.
Referring to FIG. 2, there is a hysteresis curve corresponding to the ferroelectric capacitor 100. The hysteresis curve includes net charge Q or polarization along the vertical axis and applied voltage along the horizontal axis. By convention, the polarity of the ferroelectric capacitor voltage is defined as shown in FIG. 1. A stored “0”, therefore, is characterized by a positive voltage at the plate line terminal with respect to the access transistor terminal. A stored “1” is characterized by a negative voltage at the plate line terminal with respect to the access transistor terminal. A “0” is stored in a write operation by applying a voltage Vmax across the ferroelectric capacitor. This stores a saturation charge Qs in the ferroelectric capacitor. The ferroelectric capacitor, however, includes a linear component in parallel with a switching component. When the electric field is removed, therefore, the linear component discharges, but the residual charge Qr remains in the switching component. The stored “0” is rewritten as a “1” by applying −Vmax to the ferroelectric capacitor. This charges the linear and switching components of the ferroelectric capacitor to a saturation charge of −Qs. The stored charge reverts to −Qr when the voltage across the ferroelectric capacitor is removed. Coercive points VC and −VC are minimum voltages on the hysteresis curve that will degrade a stored data state. For example, application of VC across a ferroelectric capacitor will degrade a stored “1” even though it is not sufficient to store a “0”. Thus, it is particularly important to avoid voltages near these coercive points unless the ferroelectric capacitor is being accessed. Moreover, power supply voltage across a ferroelectric capacitor must exceed these coercive voltages during a standby or sleep mode avoid data loss.
Referring to FIG. 3, there is illustrated a typical write sequence for a ferroelectric memory cell as in FIG. 1. Initially, the bit line (BL), word line (WL), and plate line (PL) are all low. The upper row of hysteresis curves illustrates a write “1” and the lower row represents a write “0”. Either a “1” or “0” is initially stored in each exemplary memory cell. The write “1” is performed when the bit line BL and word line WL are high and the plate line PL is low. This places a negative voltage across the ferroelectric capacitor and charges it to −Qs. When plate line PL goes high, the voltage across the ferroelectric capacitor is 0 V, and the stored charge reverts to −Qr. At the end of the write cycle, both bit line BL and plate line PL go low and stored charge −Qr remains on the ferroelectric capacitor. Alternatively, the write “0” occurs when bit line BL remains low and plate line PL goes high. This places a positive voltage across the ferroelectric capacitor and charges it to Qs representing a stored “0”. When plate line PL goes low, the voltage across the ferroelectric capacitor is 0 V, and the stored charge reverts to Qr representing a stored “0”.
A read operation is illustrated at FIG. 4 for the ferroelectric memory cell at FIG. 1. The upper row of hysteresis curves illustrates a read “0”. The lower row of hysteresis curves illustrates a read “1”. Word line WL and plate line PL are initially low. Bit lines BL are precharged low. At time t0 bit line precharge signal PRE goes low, permitting the bit lines BL to float. At time t1 word line WL goes high and at time t2 plate line PL goes high. This permits each memory cell to share charge with a respective bit line. A stored “1” will share more charge with parasitic bit line capacitance CBL and produce a greater bit line voltage than the stored “0” as shown at time t3. A reference voltage (not shown) is produced at each complementary bit line of an accessed bit line. This reference voltage is between the “1” and “0” voltages. Sense amplifiers are activated at time t3 to amplify the difference voltage between the accessed bit line and the complementary bit line. When respective bit line voltages are fully amplified, the read “0” curve cell charge has increased from Qr to Qs. By way of comparison, the read “1” data state has changed from a stored “1” to a stored “0”. Thus, the read “0” operation is nondestructive, but the read “1” operation is destructive. At time t4, plate line PL goes low and applies −Vmax to the read “1” cell, thereby storing −Qs. At the same time, zero voltage is applied to the read “0” cell and charge Qr is restored. At the end of the read cycle, signal PRE goes high and precharges both bit lines BL to zero volts or ground. Thus, zero volts is applied to the read “1” cell and −Qr is restored.
Referring now to FIG. 5, a pulse sensing read operation is illustrated for a ferroelectric memory circuit. The read operation begins at time t0 when precharge signal PRE goes low, permitting the bit lines BL to float. Word line WL and plate line PL are initially low, and bit lines BL are precharged low. At time t1, word line WL goes high, thereby coupling a ferroelectric capacitor to a respective bit line. Then plate line PL goes high at time t2, thereby permitting each memory cell to share charge with the respective bit line. The ferroelectric memory cells share charge with their respective bit lines BL and develop respective difference voltages. Here, V1 represents a data “1” and V0 represents a data “0”. Plate line PL then goes low prior to time t3, and the common mode difference voltage goes to near zero. The difference voltage available for sensing is the difference between one of V1 and V0 at time t3 and a reference voltage (not shown) which lies approximately midway between voltages V1 and V0 at time t3. The difference voltage is amplified at time t3 by respective sense amplifiers and full bit line BL voltages are developed while the plate line PL is low. Thus, the data “1” cell is fully restored while plate line PL is low and the data “1” bit line BL is high. Subsequently, the plate line PL goes high while the data “0” bit line BL remains low. Thus, the data “0” cell is restored. The plate line PL goes low at time t4, and precharge signal PRE goes high at time t5. The high level of precharge signal PRE precharges the bit lines to ground or Vss. The word line WL goes low at time t6, thereby isolating the ferroelectric capacitor from the bit line and completing the pulse sensing cycle.
Each of the foregoing read, write, and restore operations of the ferroelectric memory induce retained polarization domains within the ferroelectric capacitor 100. This is particularly true when a maximum electric field is applied to the ferroelectric capacitor at +/−Vmax. This phenomenon is often referred to as imprinting and may degrade the memory cell (FIG. 1) signal margin when reading an opposite data state. For example, when a “0” is frequently written to the memory cell followed by writing a “1”, residual charge may remain more positive than −Qr (FIG. 2), thereby degrading the “1” signal margin. Likewise, when a “1” is frequently written to the memory cell followed by writing a “0”, residual charge may remain more negative than Qr, thereby degrading the “0” signal margin. The present invention is directed to avoiding these and other disadvantages as will be discussed in detail.